Method for laser annealing to form an epitaxial growth layer

ABSTRACT

A method for forming an epitaxial layer in a capacitor over interconnect structure, includes selecting a laser having a suitable wavelength for absorption at a seeding layer/annealing layer interface of the capacitor over interconnect structure, and directing laser energy from the selected laser at the capacitor over interconnect structure. The laser energy anneals a feature of the capacitor over interconnect structure to form an epitaxial layer. The annealing is accomplished at a temperature below about 450° C. The selected laser can be an excimer laser using a pulse extender. The capacitor over interconnect structure can be a ferroelectric capacitor formed over a conventional CMOS structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/794,357, filed on Mar. 4, 2004, which claims priority under 35 U.S.C.§ 119(e) from U.S. Provisional Application No. 60/469,054, filed on May7, 2003. The disclosure of the prior application from which priority isclaimed is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor manufacturing,and more particularly to a method of laser annealing to obtain epitaxialgrowth.

2. Description of the Related Art

Ferroelectric Random Access Memory (FeRAM) is an ideal memory for Systemon Chip (SoC) applications because of its low operating voltage, lowpower consumption and high writing speed. Manufacture of FeRAMstructures, however, is problematic with realized backend process damageto the ferroelectric capacitor and contamination due to ferroelectricmaterials.

One approach to FeRAM manufacture is known as a COI (Capacitor OverInterconnect) process. In the COI process, the ferroelectric capacitoris fabricated after the CMOS interconnect process. The conventional CMOSprocess is undisturbed and thus integration of the ferroelectriccapacitor becomes straightforward.

One challenge of the COI process is to achieve perovskite ferroelectricphase below about 450° C. Although several attempts have been reported,all generally suffer from either failing to achieve a low enoughtemperature, or degraded remnant polarization. What is therefore neededis a method to achieve the requisite material state at a low enoughtemperature and without degraded remnant polarization.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills these needs by providing amethod to grow an epitaxial layer by laser annealing. Embodiments of thepresent invention can be easily applied to semiconductor manufacturing,thin-film-transistor liquid-crystal-display (TFT-LCD) manufacturing,etc. An exemplary implementation for embodiments of the presentinvention includes the forming of an epitaxial growth ferroelectriccapacitor using this invention for embedded FeRAM applications. Thepresent invention can be implemented in numerous ways, including as aprocess, an apparatus, a system, a device, or a method.

In one embodiment, a method for forming an epitaxial layer in acapacitor over interconnect structure is provided. The method includesselecting a laser having a suitable wavelength for absorption at aseeding layer/annealing layer interface of the capacitor overinterconnect structure. The method further includes directing laserenergy from the selected laser at the capacitor over interconnectstructure. The laser energy anneals a feature of the capacitor overinterconnect structure to form an epitaxial layer.

In another embodiment, a method of selecting a laser for laser annealinga multi-layer feature is provided. The method includes determining atransmission, reflection, and absorption of laser energy across awavelength spectrum for an uppermost layer of the multi-layer feature.The method further includes determining a transmission, reflection, andabsorption of laser energy across a wavelength spectrum for a next layeradjacent to the uppermost layer of the multi-layer feature. Anabsorption percentage of laser energy across a wavelength spectrum at aninterface between the uppermost layer and the next layer adjacent to theuppermost layer of the multi-layer feature is then calculated, and alaser is selected based on the calculated absorption percentage at aparticular wavelength.

In a further embodiment, a method of making a capacitor overinterconnect structure is provided. The method includes providing aninterconnect structure, and fabricating a ferroelectric capacitordisposed over the interconnect structure. The fabricating of theferroelectric capacitor disposed over the interconnect structureincludes the processes of selecting a laser having a suitable wavelengthfor absorption at a seeding layer/annealing layer interface of theferroelectric capacitor, and directing laser energy from the selectedlaser at the ferroelectric capacitor. The laser energy anneals a featureof the ferroelectric capacitor to form an epitaxial layer.

The advantages of the present invention over the prior art are numerous.To form an epitaxial growth layer by a low temperature process isimportant because this process is compatible with conventional CMOSprocesses or other fields, such as TFT-LCD. Because laser-annealingtechnique has an ultra-short annealing time (lower than 0.1 ms), it willnot damage inner conventional CMOS interconnects and substrates.

Embodiments of the present invention provide a laser-annealing method toobtain a low temperature epitaxial growth layer. Embodiments of theinvention also provide a method to select and to control a suitablelaser to obtain or fabricate a low temperature epitaxial growth layer.Embodiments of the present invention can be widely implemented in manyfields that utilize, or could benefit from the utilization of theprocess of forming an epitaxial layer at low temperature.

Other advantages of the invention will become apparent from thefollowing detailed description, taken in conjunction with theaccompanying figures, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute partof this specification, illustrate exemplary embodiments of the inventionand together with the description serve to explain the principles of theinvention.

FIG. 1 is a graph showing the plots of the transmission, reflection, andabsorption percentages across a spectrum of wavelengths for PZT, inaccordance with one embodiment of the present invention.

FIG. 2 is a graph showing the plots of the transmission, reflection, andabsorption percentages across a spectrum of wavelengths for LNO, inaccordance with one embodiment of the present invention.

FIG. 3 is a graph showing a plot of the resulting calculation for theabsorption expressed as a percentage at the PZT/LNO interface acrossessentially the same laser wavelength spectrum used in FIGS. 1 and 2, inaccordance with one embodiment of the invention.

FIG. 4 illustrates a first step of the process flow, which is to deposita PZT film on an LNO seeding layer at room temperature, in accordancewith one embodiment of the invention.

FIG. 5 shows energy from the selected laser system applied to theexemplary structure to heat the PZT/LNO interface, in accordance withone embodiment of the invention.

FIG. 6 illustrates the exemplary structure with the PZT epitaxiallygrown on LNO seeding layer, in accordance with an embodiment of theinvention.

FIG. 7 illustrates a schematic of a pulse extender utilized to achievethe desired conditions, in accordance with one embodiment of the presentinvention.

FIG. 8 shows a structure having an amorphous PZT layer formed over thetop of a multi-layer feature resulting from etching damage or otherprocess anomaly.

FIG. 9 illustrates laser annealing of structure shown in FIG. 8, inaccordance with one embodiment of the present invention.

FIG. 10 shows the multi-layer feature of FIGS. 8 and 9 following thelaser annealing, in accordance with an embodiment of the invention.

FIG. 11A shows a graph of remnant polarization before laser annealingwith an excimer laser.

FIG. 11B shows a graph of remnant polarization after laser annealingwith an excimer laser.

FIG. 12 shows a capacitor over interconnect structure formed inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An invention for forming an epitaxial growth layer by a low temperature,laser annealing process is described. In preferred embodiments, asuitable laser is selected and then utilized to fabricate, for example,COI structures. In the following description, numerous specific detailsare set forth in order to provide a thorough understanding of thepresent invention. It will be understood, however, to one skilled in theart, that the present invention may be practiced without some or all ofthese specific details. In other instances, well known processoperations have not been described in detail in order not tounnecessarily obscure the present invention. The terms “about” and“approximately,” as used herein, denote a value within +/−10% of thereferenced value.

As is known, laser annealing can heat a surface to high temperaturewhile leaving a bulk of materials at low temperature. Such propertiescast laser annealing as an ideal technique to fabricate COI structures.In one embodiment of the present invention, a suitable laser system isfirst selected. In order to have an epitaxial growth layer, theinterface between seeding layer and the layer that is to be annealed toform an epitaxial layer needs to be heated by a laser-annealingtechnique because the activation energy of heterogeneous nucleation atthe interface between the seeding layer and the annealing layer is lowerthan that of homogenous nucleation in the annealing layer.

First, the transmission and reflection coefficients, which are expressedin one embodiment as percentages of a total laser energy, of the seedinglayer and of the layer that will be crystallized are measured and thencompared to laser wavelengths. It is thereby possible to determine adesired wavelength to achieve a maximum absorption at the seedinglayer/annealing layer interface. The laser or laser system with thisdesired wavelength can then be selected for use to form an epitaxialgrowth layer.

FIGS. 1 and 2 illustrate this determination of desired wavelength for anappropriate laser or laser system using an exemplary Pb(Zr, Ti)O₃, alsoknown as PZT, epitaxial layer and a seeding layer of LaNiO₃, also knownas LNO. FIG. 1 is a graph 100 showing the plots of the transmission 102,reflection 104, and absorption 106 percentages 108 across a spectrum ofwavelengths 110 for PZT, in accordance with one embodiment of thepresent invention. FIG. 2 is a graph 120 showing the plots of thetransmission 122, reflection 124, and absorption 126 percentages 128across a spectrum of wavelengths 130 for LNO, in accordance with oneembodiment of the present invention. The relationships between and amongtransmission, reflection, and absorption, of PZT and LNO are illustratedin FIGS. 1 and 2.

Referring to FIGS. 1 and 2, assume the incident laser energy is equal to100%. As is known, Transmission (T)+Reflection (R)+Absorption (A)=100%.A known value of, or the ability to measure, any two of the T, R, and Avalues results in the ability to easily determine the third value. Byway of example, T is typically known or easily measured, and R is easilymeasured, and therefore A is easily calculated. In FIG. 1, Transmission,Reflection and Absorption of PZT, also denoted in the instantapplication by T_(PZT), R_(PZT) and A_(PZT), respectively, are shownexpressed in percentages 108 plotted across a wavelength 110 spectrum.The same properties, T, R, and A, of LNO are determined in the samemanner as described above for PZT, and are similarly describedthroughout the instant application as T_(LNO), R_(LNO) and A_(LNO),respectively. If a structure is comprised of PZT (top layer)/LNO(seeding layer) in accordance with one embodiment of the presentinvention, then (T_(PZT)*A_(LNO)) will be absorbed at the interface ofPZT and LNO. In another embodiment of the invention, the LNO seedinglayer is formed over or on the top of PZT, and then the interface isannealed. It should be appreciated that epitaxial growth can also beachieved in the PZT layer. The calculation to determine the absorptionat the interface of LNO and PZT in an embodiment in which the LNOseeding layer is formed over or on the top of PZT is (T_(LNO)*A_(PZT)).

The absorption at the interface of LNO and PZT, together with the power,wavelength, and other characteristics of the laser are then used toselect a proper laser for a particular application. In one embodiment,this is an approximate calculation, however it is sufficient todetermine a suitable laser for particular materials or applications.

FIG. 3 is a graph 150 showing a plot 152 of the resulting calculationfor the absorption expressed as a percentage 154 at the PZT/LNOinterface across essentially the same laser wavelength 156 spectrum usedin FIGS. 1 and 2, in accordance with one embodiment of the invention.The plot 152 of the absorption percentage 154 of laser energy at the PZTand LNO interface of the exemplary structure as shown in FIG. 3 isobtained by calculating (T_(PZT)*A_(LNO)) across the wavelength 156spectrum in accordance with an embodiment of the invention. In oneembodiment of the invention, a maximum absorption of laser energy at theinterface between the seeding layer and the annealing layer is preferredfor the laser annealing process. In the exemplary PZT/LNO structure, awavelength 156, from 385 nm to 800 nm may be suitable, as illustrated inFIG. 3, and is selected for this exemplary application.

A detailed process flow according to one embodiment of the invention isillustrated in FIGS. 4-6. FIG. 4 illustrates a first step of the processflow, which is to deposit a PZT film 166 on an LNO seeding layer 164 atroom temperature, in accordance with one embodiment of the invention.FIG. 4 shows an exemplary structure 160 having a substrate 162 overwhich is deposited an LNO seeding layer 164. The PZT film 166 isdeposited over the LNO seeding layer 164, in accordance with oneembodiment of the invention. In one embodiment of the invention, the LNOseeding layer 166 has a thickness ranging from approximately 50 nm toapproximately 150 nm, and the PZT film 166 has a thickness ranging fromapproximately 100 nm to approximately 200 nm. The PZT film 166,deposited at room temperature, is preferably amorphous. The PZT/LNOinterface is shown at 168.

Next, as illustrated in FIG. 5, a suitable laser system 172, selected inone embodiment in accordance with a desired absorption at the PZT/LNOinterface 168 as described above, is utilized to form epitaxial growthin the PZT layer 166. FIG. 5 shows energy 170 from the selected lasersystem 172 applied to the exemplary structure 160 to heat the PZT/LNOinterface 168, in accordance with one embodiment of the invention.

FIG. 6 illustrates the exemplary structure 160 with the PZT 166epitaxially grown on LNO seeding layer 164, in accordance with anembodiment of the invention. FIG. 6 shows the exemplary structure 160with the substrate 162, over which has been formed the LNO layer 164.The PZT film 166 was deposited over the LNO layer 164 at roomtemperature, and then heated with a selected laser 172 to heat thePZT/LNO interface 168 as described above, and resulting in the PZT layer166 now in a perovskite phase.

The specifications of a suitable laser system and annealing conditionsin accordance with one embodiment of the present invention are as shownbelow in Table 1. It should be noted that the laser pulse duration canbe assisted by a pulse extender to achieve the desired pulse duration inaccordance with known laser processes and procedures, and as describedin further detail below. TABLE 1 Laser specifications or annealingconditions Range Wavelength of lasers 358˜800 nm Laser pulse duration0.5 μs˜100 μs (Can be assisted by pulse extender) Laser energy densityper pulse 250 mJ/cm²˜10000 mJ/cm² (mJ/cm²) Number of shots 1˜10000 shotsSubstrate assisted temperature Room temperature ˜450° C.

In one embodiment of the invention, an excimer laser and apulse-extending apparatus and technique can be utilized to achieve thedesired annealing process. A typical pulse from excimer lasers is tooshort in duration (10-30 ns) to achieve desired annealing conditionsbecause it does not allow the film adequate time to transform into theperovskite phase. With an extended pulse, top and bottom portions of thePZT film are both heated, and for a much longer time than innon-pulse-extended processes. Therefore, an extended pulse offers bothmore uniform heating and a longer heating time to completecrystallization into the desired perovskite phase.

FIG. 7 illustrates a schematic 200 of an exemplary pulse extenderutilized to achieve the desired conditions, in accordance with oneembodiment of the present invention. In FIG. 7, a pulse extender 200 isillustrated as a simple optical apparatus that can be implemented in achamber or box 202 having banks of mirrors 204 along parallel, opposingsides. Nitrogen is introduced 208 into box 202 and maintained at adesired and appropriate pressure, temperature, etc. Nitrogen exits box202 through nitrogen output 218. A plurality of partial reflectors 206is disposed as illustrated between the parallel opposing banks ofmirrors 204. An input beam 212 is introduced into the chamber 202through an input lens 214. Using a pulse extender 200. the effectivepulse duration of a laser, for example, is increased by extending thepath lengths of laser beam 212 after passing through each of theplurality of partial reflectors 206. Exemplary pulse extending apparatusand techniques such as that illustrated in FIG. 7 are available fromExitech Ltd., Oxford, United Kingdom.

An exemplary excimer laser is a KrF (248 nm) excimer laser with anominal pulse width of 25 ns. The 25 ns pulse width of the KrF excimerlaser can be stretched or extended to approximately 374 ns using pulseextension techniques and apparatus. In so doing, the resulting pulseduration is significantly increased over non-extended pulse duration,and the resulting extended pulse is suitable for laser annealing byproviding the annealing layer sufficient time to crystallize into anepitaxial layer.

FIG. 8 shows a structure 250 having an amorphous PZT layer 258 formedover the top of a multi-layer feature resulting from etching damage orother process anomaly. As shown in FIG. 8, the multi-layer featureincludes a substrate 252 over which is formed a bottom electrode layer254. A PZT film 256 is formed over the bottom electrode layer 254, andan amorphous PZT layer 258 has formed over PZT film 256. In oneembodiment, bottom electrode layer 254 ranges from approximately 50 nmto approximately 150 nm in thickness, and is comprised of such exemplarymaterials as platinum (Pt), iridium (Ir), iridium oxide (IrO₂), LNO, orother similar material. PZT film 256, in one embodiment, is formed to athickness ranging from approximately 5 nm to approximately 300 nm. Inthe illustrated embodiment, PZT film 256 is formed in a perovskitephase, and PZT layer 258 is amorphous and has a thickness approachingzero and ranging to approximately 70 nm. An interface between PZT film256 and amorphous PZT layer 258 is identified at 260. In accordance withan embodiment of the invention, structure 250 can be laser annealed byan excimer laser with a pulse extender.

FIG. 9 illustrates laser annealing of structure 250 shown in FIG. 8, inaccordance with one embodiment of the present invention. As shown inFIG. 9, an excimer laser 264, having pulse extending apparatus andtechniques to achieve a desired wavelength as described above andselected in accordance with the process illustrated and described abovein reference to FIGS. 1-3, is directed at structure 250. In theillustrated embodiment, absorption is calculated for the interfacebetween PZT film 256 and amorphous PZT layer 258 at 260. Excimer laserenergy 262 is directed at structure 250 to heat interface 260 betweenperovskite PZT film 256 and amorphous PZT layer 258.

FIG. 10 shows structure 250 of FIGS. 8 and 9 following the laserannealing, in accordance with an embodiment of the invention. Theamorphous PZT layer 258 shown in FIG. 9 has been crystallized intoperovskite phase by application of an excimer laser.

In accordance with embodiments of the present invention, remnantpolarization is markedly improved by excimer laser annealing. Thedigital signal of FeRAM is stored in the ferroelectric capacitor, andthe magnitude of the sensing window of FeRAM is proportional to theremnant polarization of the ferroelectric capacitor. A laser annealingprocess in accordance with embodiments of the present invention cantherefore improve the sensing window of FeRAM. FIGS. 11A and 11Billustrate the realized improvement in remnant polarization.

FIG. 11A shows the remnant polarization before laser annealing with anexcimer laser, and FIG. 11B shows the remnant polarization after laserannealing with an excimer laser. The remnant polarization of aferroelectric capacitor is dependent on the amount of perovskite phaseof the ferroelectric film, such as PZT. In FIG. 11A, the ferroelectricfilm, PZT, is not fully crystallized into perovskite phase so that theremnant polarization shown in FIG. 11A is smaller than that shown inFIG. 11B.

As shown in FIG. 11B, the ferroelectric film, PZT, is almost fullycrystallized into perovskite phase by using laser annealing techniques,in accordance with embodiments of the present invention, so that theremnant polarization shown in FIG. 11B is larger than that shown in FIG.11A.

The specifications of laser system and annealing conditions for thecrystallization of a thin amorphous PZT layer formed over a perovskitePZT film are as shown below in Table 2. TABLE 2 Laser specifications orannealing conditions Range Wavelength of lasers 157˜351 nm Laser pulseduration 0.1 μs˜10 μs (Assisted by pulse extender) Laser energy densityper pulse 150 mJ/cm²˜1000 mJ/cm² (mJ/cm²) Number of shots 1˜10000 shotsSubstrate assisted temperature Room temperature ˜450° C.

As described above, an exemplary implementation for embodiments of thepresent invention includes utilization to form COI structures. FIG. 12shows a capacitor over interconnect structure 300 formed in accordancewith an embodiment of the present invention. As shown in FIG. 12, theexemplary COI structure 300 includes a lower portion 302 havingstructures formed by conventional CMOS processes. Ferro-capacitor 304 isformed in accordance with embodiments of the present invention asdescribed throughout the instant application. In one embodiment,ferro-capacitor 304 is formed after the fabrication of the conventionalCMOS structures in region 302, leaving the conventional CMOS processundisturbed. An epitaxial layer has been grown by laser annealing asdescribed herein, to achieve the desired perovskite ferroelectric phasebelow 450° C., and enabling fabrication of FeRAM structures.

In summary, embodiments of the present invention provide for theformation of epitaxial growth layers by a laser annealing technique.Embodiments of the present invention have been illustrated by exemplarymulti-layer structures such as a PZT film deposited over LNO seedinglayer, and it should be understood that the structure is exemplary andnot limiting or exclusive. In one embodiment, the low temperature COIprocess is suitable for embedded FeRAM for SoC application Embodimentsof the method also can be applied to other crystallization processes,such as BLT, SBT, and other similar materials and processes.Laser-annealing recipes of amorphous PZT formed over an LNO seedinglayer are shown in table 1. The laser-annealing recipes shown in table 2are for the crystallization of thin amorphous PZT layer on the top of aperovskite PZT film.

Embodiments of the present invention provide for the formation of anepitaxial layer without damaging underlying CMOS interconnects orsubstrate. Although the foregoing invention has been described in somedetail for purposes of clarity of understanding, it will be apparentthat certain changes and modifications may be practiced within the scopeof the appended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

1. A method of selecting a laser for laser annealing a multi-layerfeature, comprising: determining a transmission, reflection, andabsorption of laser energy across a wavelength spectrum for an uppermostlayer of the multi-layer feature; determining a transmission,reflection, and absorption of laser energy across a wavelength spectrumfor a next layer adjacent to the uppermost layer of the multi-layerfeature; calculating an absorption percentage of laser energy across awavelength spectrum at an interface between the uppermost layer and thenext layer adjacent to the uppermost layer of the multi-layer feature;and selecting a laser based on the calculated absorption percentage at aparticular wavelength.
 2. The method of claim 1, wherein the uppermostlayer of the multi-layer feature is in an amorphous phase.
 3. The methodof claim 2, wherein the uppermost layer is Pb(Zr, Ti)O₃.
 4. The methodof claim 1, wherein the next layer adjacent to the uppermost layer ofthe multi-layer feature is comprised of one of LaNiO₃, platinum,iridium, and iridium oxide.
 5. The method of claim 4, wherein the nextlayer adjacent to the uppermost layer of the multi-layer feature is in aperovskite phase.
 6. The method of claim 1, wherein the multi-layerfeature is a capacitor over interconnect structure.